Big Macs have gotten quite a bit smaller than they used to be.
Big Macs aren't nearly as big as they were a long time ago.
この場合、次のような言い方ができますよ。
ーBig Macs have gotten quite a bit smaller than they used to be.
「ビッグマックは昔よりかなり小さくなった」
to get quite a bit smaller「かなり小さくなる」
ーBig Macs aren't nearly as big as they were a long time ago.
「ビッグマックは昔ほど全く大きくない」
not nearly as ... as で「〜より全く…でない」
ご参考まで!